Cardiac arrhythmias are the result of various defects in the heart. One such defect is an extraneous stand of muscle fibre in the heart that provides an abnormal short circuit pathway for electric impulses traveling through the heart tissue. This accessory pathway often causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber, causing the heart to beat irregularly and therefore inefficiently pump blood.
Another common type of cardiac arrhythmia is ventricular tachycardia (VT), which may be a complication resulting from a heart attack or from a temporary reduction of blood supply to an area of heart muscle. VT is often caused by a tiny lesion, typically on the order of 1–2 mm, that is located close to the inner surface of the heart chamber. That lesion is often referred to as an “active site” because it does not fire in sequence with the rest of the heart muscle. VT causes the heart's normal rhythmic contraction to be altered, thereby affecting heart function. A typical symptom is rapid, inefficient heartbeats. Minimally invasive techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and also to disable the short-circuit function of these areas. According to these techniques, electrical energy shocks are applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways. The regions to be ablated are usually first determined by endocardial mapping techniques. Mapping typically involves the percutaneous introduction of a diagnostic catheter having one or more electrodes into the patient, passing the diagnostic catheter through a blood vessel (e.g. the femoral vein or aorta) and into an endocardial site (e.g., the atrium or ventricle of the heart), and inducing a tachycardia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions. When a tachycardia focus is located, as indicated in the electrocardiogram recording, it is marked by means of a fluoroscopic image so that cardiac arrhythmias at the located site can be ablated. An ablation catheter with one or more electrodes can then provide electrical energy to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will create a region of necrotic tissue to disable the malfunction caused by the tachycardia focus.
Ablation is carried out by applying energy to the catheter electrodes once the electrodes are in contact with the cardiac tissue. The energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. When RF energy (e.g. 510 KHz)is delivered between the distal tip of a standard electrode catheter and a backplate, there is a localized RF heating effect. This creates a well-defined, discrete lesion slightly larger than the tip electrode (i.e., the “damage range” for the electrode), and also causes the temperature of the tissue in contact with the electrode to rise.
Radio frequency ablation catheters are available with different types of ablation electrodes. These electrodes have different diameters and profiles depending on the intended therapeutic application, the anatomic location of the electrode, and the efficiency by which the RF energy is transmitted from the electrode in the form of heat. It is known that the current in an electrode associated with RF energy tends to have greater concentrations where the surface of the electrode is less continuous, i.e. less smooth. Such surface areas generally have an abrupt transition such as an edge. This higher concentration of electrical current can result in unequal heating at the surface of the electrode due to the nonuniform distribution of energy. At about 100° C., charring and desiccation occur which significantly changes the electrical conductivity of blood and tissue, causing an increase in the overall impedance of the electrical heating circuit and thus a diminution in the power delivered to the tissue. Charring is particularly troublesome because of the possibility that the char may be dislodged from the electrode and enter the blood stream causing serious adverse effects for the patients. When charring occurs, the electrode must be removed and cleaned before the procedure can continue.
In conventional RF ablation catheters, the electrode is provided with a temperature sensing element which is used to control the energy fed to the electrode. This system works if the sensor is placed properly at the most likely site on the electrode surface where hot spots are likely to occur. If multiple hot spots are possible, multiple temperature sensors are needed. As electrodes become more complex, the need to use multiple temperature sensors becomes a limiting factor both from the viewpoint of cost and performance and may result in compromises in electrode design to minimize the appearance of hot spots due to electrical current concentration.
Accordingly, it is an object of the invention to provide an RF ablation catheter which is less likely than prior art electrode catheters to cause charring.
A more specific object of the invention is to provide an RF ablation catheter wherein multiple temperature sensors are not required to avoid the creation of hot spots which can lead to charring.